This application is based on Japanese Patent Application No. 10-079443 filed Mar. 26, 1998 and Japanese Patent Application No. 11-69025 filed Mar. 15, 1999, the contents of which are incorporated hereinto by reference.
1. Field of the Invention
The present invention relates generally to a printing apparatus. More particularly, the invention relates to a printing apparatus which performs a plurality of scans, while relatively shifting a printing medium and a printing head for respectively predetermined amounts (multi-pass printing), to complete an image.
2. Description of the Related Art
A printing head of an ink-jet printer has a large number of nozzles aligned in a paper feeding direction. Performing a scanning operation (a primary scan), with an ink ejection while the printing head is driven to move in a direction (a primary scanning direction) different from a nozzle alignment direction, and a paper feeding operation (an auxiliary scan), for respectively predetermined amounts, repeatedly for a plurality of times, completes an image of one area. However, due to hitting position error to be caused by fluctuation of an ink ejection amount, fluctuation of a paper feeding amount, kink of the nozzles (error in forming positions) and so on, and due to ink absorption properties of a printing medium, a printed image can be formed accompanied with irregular color or stripes in a lateral direction.
As a solution for the problem set forth above, there has been known a multi-pass printing, such as two-pass printing or four-pass printing. In the four-pass printing, for example, the paper feeding amount is set at one fourth of a maximum width printed by one scan with the printing head, and one fourth of dots included within the scanning width are printed in each scan. Thus, four times scanning completes all of dots which included in one printing region of a longitudinal width corresponding to the paper feeding amount.
Connecting stripes formed, between printing regions, in the primary scanning direction and in the multi-pass printing employing with the thinned patterns, are so visually perceptible that image quality is to be degraded. In the multi-pass printing, connecting stripes in the primary scanning direction between printing regions formed by the thinned patterns becomes visually perceptible to degrade image quality. One reason for such connecting stripes is paper feeding error which constantly occurs. The feeding error in constant can be caused due to difference in thickness at paper ends, or due to error in a diameter of a paper feeding roller. An alternative reason to the paper feeding error of the connecting stripes may also be scattering of a nozzle pitch about a design value.
FIG. 1 shows a system configuration of a conventional printer.
In FIG. 1, a printer 10 is provided with interconnected components, an interface(I/F) 11, a CPU 12, a ROM 13, a RAM 14 and a print control unit 15, and performs printing in communication with a host computer(PC) 18.
The ROM 13 has a control program storage area 13a and a printing mask storage area 13b, and preliminarily stores a control program of the printer 10 and several printing masks which the printer 10 uses. The CPU 12 operates according to the control program stored in the control program storage area 13a to generate the printing masks to be used in a current printing mode. The RAM 14 has a printing buffer 14a and a printing mask storage area 14b to store printing data and the printing masks to be used in the current printing mode. The print control unit 15 controls a printing head (see FIG. 5A) having thirty-two nozzles for ink ejection.
FIG. 2 shows a side elevation of an essential part of the conventional printer.
A paper feeding device 22 is to perform the paper feeding operation. Here, the paper feeding operation means transporting of a printing medium M in an arrow A direction (the auxiliary scan). A printing head 20 is to perform the scanning operation (the primary scan). The scanning operation represents an ink ejection while shifting the printing head 20 in arrow B and/or C direction. The printer 10 carries out the multi-pass printing on the printing medium M while performing the paper feeding operation and the scanning operation repeatedly. It should be noted that relatively moving, in the two directions, of the printing medium M and the printing head 20 can accomplish the primary scan and auxiliary scan both.
The printing head 20 has a plurality of nozzles (not shown) aligned in the paper feeding direction. The printing head 20 is a so-called ink-jet head, in which a heater (not shown) is provided in a liquid path of each nozzle for ink ejection for printing, which is achieved by film boiling of the ink, within the liquid path, caused by driving the heater to generate thermal energy, based upon image data.
A printing process by the printer 10 will be explained.
At first, the host computer 18 performs designation of a printing mode via an interface. The CPU 12 retrieves the printing mask to be used in the current printing from the ROM 13 according to the designated printing mode to extend the printing mask in the printing mask storage area 14b of the RAM 14. The printer 10 receives the printing data to extend bitmap data in the printing buffer 14a.
Whenever a predetermined amount of the bitmap data is stored in the printing buffer 14a, the CPU 12 issues a printing instruction to the print control unit 15. The print control unit 15 drives the heater with the designated bitmap and the extended printing mask in order to eject the ink to the printing medium.
FIG. 3 shows one example of the conventional printing mask for accomplishing four-pass printing with the printing head.
A printing mask 30 is of a size of 32 dots, corresponding to number of nozzle in the paper feeding direction, by 36 dots in the primary scanning direction. All dots of the printing mask can be applicable to the ink ejection with combined values of (A-x, B-x, C-x, D-x) (1 less than x less than 8). The dots are arranged in such a way that an image of an area on the printing medium is completed by scanning the same area respectively with each of area A, B, C, D. A printing ratio in each area A, B, C, D is set at 25%. Namely, for hatched dots, the ink ejection is effective.
Operation of the print control unit 15 employing with the printing mask set forth above will be explained with reference to FIGS. 1 and 4.
The print control unit 15 retrieves bitmap data 400 in a printing buffer 14a and a printing mask 42 in the printing mask storage area 14b in sequential order, and an AND operation of both is carried out. In the printing mask 42, when the dot is once retrieved up to the final dot, the dot retrieving address returns to the beginning. On the basis of the AND operation result, the effectiveness of the data is determined in order to perform the ink ejection according to the effective data while scanning the printing medium M with the printing head 20.
A completing process of the image by four-pass printing while repeating the paper feeding operation by a prescribed quantity corresponding to 8 nozzles, will be explained with reference to FIGS. 5B, and 6A to 6H. FIG. 5B shows an image printing result by the multi-pass printing while employing the printing head 20, the nozzle construction of which is shown in FIG. 5A. FIGS. 6A to 6H show a forming process of the connection stripes by every scan.
The printer 10 performs the paper feeding for eight nozzles and the scanning repeatedly. Thus, four times of scan completes the image, in each of which scan one fourth of the printing data is used, respectively.
Here, it is assumed that the paper feeding operation of the printer 10 causes a paper feeding error in feeding amount in short of 1%.
Under these conditions, connecting stripes 51 of FIG. 5B are formed by a first scan scanning the upper region in the drawing to a fifth scan scanning the lower region in the drawing, through the process of FIGS. 6A to 6E. The paper feeding amount to be performed between the first scan and the fifth scan corresponds to thirty-two nozzles width caused by four times of paper feeding operation. Therefore, a shortage amount corresponding to 0.32 nozzles caused by the paper feeding operation yields an offset amount of the image to be formed by the fifth scan relative to that should be. An overlap of the offset amount, caused by the image formed in the fifth scan and the image formed in the first scan, yields connecting stripes 51.
Concerning a connection portion 52, similar connecting stripes 52 are formed through the process of FIGS. 6B to 6F in the similar mechanism. Similarly, concerning a connection portion 53, similar connecting stripes 53 are formed through the process of FIGS. 6C to 6G in the similar mechanism. Also, concerning a connection portion 54, similar connecting stripes 54 are formed through the process of FIGS. 6D to 6H in the similar mechanism.
As set forth above, density of the connecting stripes depends on the paper feeding amount with every scan during multi-pass printing. Accordingly, in order to prevent degradation of the image quality with eliminating the connecting stripes, a method of decreasing unit amount of paper feeding has been taken conventionally. For example, a method of reducing paper feeding operation to four nozzles width employing with the printing mask for feeding for four nozzles width. The method described above can make it possible to reduce the offset amount at the connection portion to be half, i.e. 0.16 nozzles.
Dispersion of an ink can be another cause of connecting stripes formation in the connection portion. In the multi-pass printing, a condition where ratios of printed dots before and after the connection portion are constantly different, is caused during printing process. While this condition is caused, dispersion of the ink from a region having greater ratio of printed dots to a region having smaller ratio of printed dots is continuously caused, coloring agent in the ink may be accumulated in the connection portion due to surface tension, or, in the alternative, the coloring agent in the ink may flow away from the connection portion. As a result, a phenomenon that color development of the ink in the connection portion becomes different from that in other regions to form the connecting stripes, is caused.
This phenomenon will be explained with reference to FIG. 5B.
Since the first scan of the printing head 20 performs printing only in the upper region of the connecting stripes 51, difference in the ratio of the printed dots is caused about this portion. The difference about this portion cannot be resolved even by performing the second, third and fourth scans, but can be resolved by the fifth scan. Therefore, during a period from the first scan to the fifth scan, the connecting stripes 51 is continuously formed.
For preventing the degradation of the image quality due to the connecting stripes formed as above, it becomes necessary to cut down the period where the difference between the ratios of the printed dots is maintained. However, in the conventional multi-scan method, it has not been possible to perform printing for resolving the difference between the ratios of the printed dots before completion of the image. Therefore, a technique to make the unit amount of paper feeding in the auxiliary scan to shorten the formation interval of the connecting stripes is used to make the connecting stripes not so visually perceptible.
However, in the prior art set forth above, printing speed can be lowered by setting the unit amount of paper feeding smaller. Therefore, there arose a problem that simply increasing number of nozzles cannot achieve rising up printing speed while outputting prints with high quality.
The present invention is worked out in view of the drawbacks in the prior art as set forth above. Therefore, it is an object of the present invention to provide a printing apparatus which can reduce connecting stripes without causing lowering of a printing speed by reducing the unit amount of paper feeding.
A printing apparatus, a printing head, a head cartridge and a control method for controlling a printing apparatus, according to the present invention accomplish that; in a printing apparatus comprising an auxiliary scanning means for relatively moving a printing head having a plurality of nozzles aligned in an auxiliary scanning direction and a printing medium substantially in the auxiliary scanning direction for a predetermined amount, and a primary scanning means for relatively moving the printing head and the printing medium in a primary scanning direction different from the auxiliary scanning direction; a plurality of images may be printed on the printing medium by the printing head employing with respective of a plurality of thinning patterns and an image on the printing medium may be formed by combining the plurality of images. The image is formed in such a manner that a plurality of boundary portions of images on the printing medium, formed adjacently with each other in the auxiliary scanning direction by respective of the thinning patterns, are located at different positions in the auxiliary scanning direction.
Hence, the above present inventions can output a high quality image at high speed while reducing a predetermined amount in shifting the printing medium, without lowering an output speed.
Furthermore, a printing apparatus according to the present invention, in performing printing operation while scanning a plurality of times by a printing head having a plurality of nozzles with thinning image data, generates a plurality of thinning patterns each having substantially the same ejection ratio and a length in the auxiliary scanning direction corresponding to the predetermined amount, which completes an image of the predetermined area by combining images formed with respective of the thinning patterns respectively; and shifts the plurality of thinning patterns in such a manner that a plurality of boundary portions of images formed by respective of the thinning patterns adjacently located with each other in the auxiliary scanning direction on the printing medium are located at different positions in the auxiliary scanning direction.
Hence, as connection portions of respective scanned images are not consistent with on the printing medium in the above present invention, a high quality image can be outputted at high speed while reducing a predetermined amount in shifting the printing medium, without lowering the output speed.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of the embodiments thereof taken in conjunction with the accompanying drawings.